1. Field of the Invention
The present invention relates to a malfunction detecting circuit implemented in a power converting system and a detecting method thereof, and more particularly, to a malfunction detecting circuit for detecting malfunction of a current sensing resistor in a power converting system, and a detecting method thereof.
2. Description of the Prior Art
FIG. 1 is a diagram of a conventional fly-back power converter 100. When the power switch Q1 of the fly-back power converter 100 is turned on, the energy from the rectified DC input voltage Vin is stored in the primary winding LP of the transformer T. The stored energy is then passed to the secondary winding LS to form the output voltage VOUT when the power switch Q1 is turned off. The conducting of power switch Q1 is controlled by a pulse width modulation (PWM) signal outputted by the PWM control chip 110, which adjusts the duty cycle of the PWM signal in order to generate the desired output voltage VOUT.
Sometimes malfunction, such as mechanical breakdown, improper operation, or any other accident, occurs and renders the resistance of the current sensing resistor Rcs almost zero, accidentally short the source pin of power switch Q1 to ground. Thus, the current sensing pin CS can no longer detect the over current situation of the primary winding current IP, and the control chip 110 may conduct the power switch Q1 at the maximum duty cycle, resulting in an unexpectedly-increasing output voltage VOUT, which may adversely affect the operation of the fly-back power converter 100. One solution to the detection of the malfunction is to detect the Vcc voltage provided by the auxiliary winding LAUX of the transformer T, and to determine if the output voltage VOUT is too high according to the detecting result. When the control chip 110 detects that the Vcc voltage is higher than a predefined over voltage protection threshold, it determines the output voltage VOUT is too high, and an over voltage protection mechanism should be activated.
In practice, the predefined over voltage protection threshold is much higher than the voltage of Vcc under normal operation, however. It will be complex and difficult to design the coil ratio of the primary winding LP to the auxiliary winding LAUX while giving consideration to both normal operation and over voltage protection. Moreover, when the output voltage VOUT initially increases from zero voltage level, the Vcc voltage has already been away from zero voltage level since it obtains part of its power directly from the input AC voltage VAC through a resistor R1. Hence, when the primary winding LP is suddenly stopped conducting current, the diode DSN at the secondary side becomes conductive earlier than the diode DA at the auxiliary side, causing that the secondary winding LS obtains power stored in the primary winding LP earlier than the auxiliary winding LAUX does. As a result, it may be deemed late to enable the over voltage protection only when Vcc voltage is detected to be higher than the predefined over voltage protection threshold, because the output voltage VOUT may be too high and cause damage to the circuits coupled to the output end of the fly-back power converter 100.